Mechanical performances of foamed polypropylene composites hybrid-reinforced by short fibers

The polypropylene (PP) pre-foamed granules containing short fibers were fabricated by a combination process of the additives pre-mixing and the secondary extrusion, and then PP foamed composites hybrid-reinforced by short carbon fibers (SCF) and short glass fibers (SGF) were prepared via a post-foaming process in designed dies. The effects of the mass ratio of SCF to SGF under constant fibers loading, the interfacial properties between reinforcement fibers and resin, and the apparent density of foams on the foamed result and mechanical performances of PP foamed composites were investigated. The results indicate that the high temperature melt strength of PP is improved by co-addition of SGF and SCF, which results in the formation of PP foams with fine and closed cell structure. The strength and modulus of the foamed PP composites are improved by the hybrid reinforcing of SGF and SCF and are better than those of monolithic SGF or SCF reinforced foamed PP. When the total fibers mass fraction keeps 15% and the mass ratio of SGF to SCF is 1 ∶ 1, the flexural strength and compressive modulus of foamed PP composites are optimal, and peak values of impact toughness and compressive strength are obtained at the ratio of SGF to SCF as 3 ∶ 1. The apparent density of PP foamed composites influences significantly the impact toughness and compressive strength of PP foamed composites, increasing from 4.29kJ/m² and 6.57MPa at 0.32g/cm³ to 17.87kJ/m² and 20.57MPa at 0.45g/cm³, respectively.